1. Field of the Invention
This invention relates to a gas-liquid contacting device for efficiently bringing a gas into contact with a slurry solution and also relates to a wet flue-gas desulfurization system using the contacting device.
2. Description of the Related Art
Among recent systems for wet flue-gas desulfurization, the in-situ oxidation system has come to be dominant. That type is so called because air is introduced into a tank of an absorption tower where it is contacted with a slurry solution (of a calcium compound such as limestone) that has absorbed sulfur dioxide from flue gas and carries out oxidation, dispensing with an oxidation tower. With a system of the type, how efficiently the air and the slurry solution are brought into contact is a key to the saving of air and power consumption, speedup of the treatment, reduction of the tank size, and other improvements. The means for supplying air to the tank for contact with the slurry solution, that is, a gas-liquid contacting device, comprises a mere array of air supply pipes mounted in the tank to effect bubbling. The means is not fully satisfactory in performance compared with oxidation tower of the pressure type and the like. With this in view, the present applicant has more recently developed and put into use a gas-liquid contacting device of a so-called arm rotating type, wherein air is supplied behind agitation bars turning inside a tank, and also a wet flue-gas desulfurization system using the contacting device.
FIG. 3 schematically shows the arrangement of a wet lime-gypsum desulfurization system incorporating the arm rotating type gas-liquid contacting device. The contacting device comprises agitator bars 4 suspended by a hollow rotating shaft 3 in a tank 2 of an absorption tower 1 so as to be driven for horizontal turning by a motor not shown, air supply pipes 5 branched from hollow rotating shaft 3, with open ends 5a extended beneath agitator bars 4, and a rotary joint 6 connecting the upper end of hollow rotating shaft 3 to an air source not shown. Hollow rotating shaft 3 is caused to run while air is being forced into the hollow shaft, whereby air C is supplied to gas phase regions being formed behind turning agitator bars 4. The vortical forces that result from the rotation of agitator bars 4 shred the trailing end portions of the gas phase regions, thus producing numerous fine bubbles substantially uniform in size. This phenomenon promotes efficient contact between air and the absorbent slurry solution that has absorbed sulfur dioxide in tank 2, until the slurry is totally oxidized and gypsum as a by-product is obtained.
In the system shown, untreated flue gas A is led into a flue gas inlet 1a of absorption tower 1, brought into contact with an absorbent slurry solution being sprayed from a header pipe 8 by a recirculation pump 7, freed from sulfur dioxide, and then is discharged as treated flue gas B from a flue gas outlet 1b. The absorbent slurry solution that has been sprayed from header pipe 8 flows down while absorbing sulfur dioxide from the flue gas, via a packing section 9, into tank 2. Inside the tank the slurry solution is stirred by agitator bars 4, oxidized by contact with the countless bubbles that have stemmed from the shredding phenomenon, and then converted into gypsum by a neutralization reaction. Principal reactions that take place during this treatment are expressed by the following reaction formulas (1) to (3). ##STR1##
Thus, inside the tank 2, gypsum and a small amount of limestone as the absorbent are suspended. They are drawn out of the tank by a slurry pump 10 and led to a thickener 11, and a resulting concentrated solution D is fed by another slurry pump 11a to a solid-liquid separator 12, where it is filtered and a cake with a low water content is taken out as gypsum E. Meanwhile a supernatant fluid F from thickener 11 and the filter drain from solid-liquid separator 12 are both sent to a filtrate tank 13, where limestone G is added and the mixture as an absorbent slurry solution is fed back to tank 2 by a slurry pump 14.
To maintain a high desulfurization rate and gypsum purity during operation, the sulfur dioxide concentration in the untreated flue gas A, pH in the tank, and other parameters are monitored by sensors, and on the basis of the monitored information the supply rates of limestone and absorbent slurry solution and the like are suitably adjusted by controls not shown. The open ends 5a of air supply pipes 5 are extended downward, usually about 200 mm beneath the underside of agitator bars 4. The extended end portions allow the splash that may gain entrance into the pipes during operation to flow down, thus preventing scale deposition on the inner walls of air supply pipes 5 during operation for long periods.
In the gas-liquid contacting device of the construction described above, it is necessary that the height H.sub.2 of agitator bars 4 from the bottom of tank 2 should be above the height H.sub.t of a deposit of solids in the slurry solution that would settle down in tank 2 upon stoppage of agitator bars 4 or recirculation pump 7 (the latter height being hereinafter called "the slurry deposit height"). Should agitator bars 4 be buried in a deposit of the solids in the case of an emergency stop due to some trouble of the flue gas purification system, they might become unable to restart breaking down the resistance of the deposit. If this possibility were to be precluded by providing an extra power supply to keep agitator bars 4 running in an emergency or by using an enhanced driving torque for the bars 4, the cost would be substantial.
The construction of the prior art gas-liquid contacting device has called for tanks larger than necessary and cumbersome to maintain. The bubbles produced by the shredding phenomenon come in contact with the slurry solution as they ascend from the vicinity of agitator bars 4 to the liquid level. It means that the effective oxidation volume represents the region between the rotating position of agitator bars 4 and the liquid level; the tank bottom portion below agitator bars 4 contributes practically nothing to the gas-liquid contact, or the oxidation reaction. For the maintenance of absorption tower 1, the slurry solution in tank 2 must at times be discharged, with stirring, by the slurry pump. When the liquid level has fallen to the slurry deposit height, agitator bars 4 no longer stir but run idle, and eventually the deposit of solids on the bottom of tank 2 has to be scraped out by human hands.
These problems are looming larger to the art since the recent tendency toward higher sulfur contents in fuels has entailed increasing sulfur dioxide concentrations in flue gases. In order to obtain high purity gypsum from the flue gases by a convenient filtration treatment or the like while keeping a high desulfurization rate, it is necessary for a flue gas desulfurization system to set the solids concentration in tank 2 usually to a high level of about 30% by weight. For example, when the height H.sub.1 of the liquid level of a slurry solution as measured from agitator bars 4 is to be set to about 4 meters so as to secure an effective oxidation volume, the height H.sub.2 of agitator bars 4 from the tank bottom must be about 2 meters, or well above the slurry deposit height H.sub.t (H.sub.t =0.3.times.H, or about 1.8 meters in this case), which add up to the liquid level height H of about 6 meters, necessitating a sufficiently deep and large tank 2 to accommodate them all. When the slurry solution must be discharged for the maintenance of the system, the depth of about 2 meters cannot be stirred and the bottom solids layer at least 0.6 meter thick must be manually scraped out. The tank being as large as about 10 meters in diameter, it is plain hard labor, adding much to the costs and time of maintenance.